Magnetically-switchable variable resistance network

ABSTRACT

A VOLTAGE GENERATOR VARIABLE BY STEPS IN RESPONSE TO ROTATION OF A MANUALLY-OPERABLE CONTROL MEMBER, WHICH CONNECTS TO VARIOUS TAPS ON A VOLTAGE DIVIDED THROUGH SUITABLE CURRENT-LIMITING RESISTORS BY MEANS OF A MAGNETICALLY-OPERABLE SWITCH, THE LATTER OPERATING ON ONE VOLTAGE DIVIDER UNIT WHEN ON A SINGLE CONTACT AND ON TWO ADJACENT UNITS WHEN BETWEEN CONTACTS.

Jan. 26, 1971 A. L. ISAACS 3 4 Mcxnncmfiswmcamw VARIABLE RESISTANCE NETWORK original Filed Oct. 2:. 1967 2 Sheets-Sheet 1 ANTHONY LEONARD ISAACS INVENTOR ATTORNEY Jan. 26,1971 v A; L. ISAACS 3,559,144

' MAGNETICALLY-SWITCHABLE VARIABLE RESISTANCE NETWORK Original Filed Oct. 2:. @967 2 Sheets-Sheet a ANTHONY LEONARD ISAACS INVENTOR B Y mu. I,

TTORNE'Y United States Patent 3,559,144 MAGNETICALLY-SWITCHABLE VARIABLE RESISTANCE NETWORK Anthony Leonard Isaacs, London, England, assignor to Thorn Electronics Limited, London, England Continuation of application Ser. No. 677,277, Oct. 23, 1967. This application Sept. 22, 1969, Ser. No. 860,104 Claims priority, application Great Britain, Oct. 21, 1966, 47,432/ 66 Int. Cl. H01c /02 US. Cl. 338173 2 Claims ABSTRACT OF THE DISCLOSURE A voltage generator variable by steps in response to rotation of a manually-operable control member, which connects to various taps on a voltage divider through suitable current-limiting resistors by means of a magnetically-operable switch, the latter operating on one voltage divider unit when on a single contact and on two adjacent units when between contacts.

This application is a continuation of application Ser. No. 677,277, filed Oct. 23, 1967.

The present invention relates to variable impedance apparatus which may, for example, be used as a fader for stage or studio lighting.

According to the present invention there is provided a variable impedance apparatus for use in controlling the brightness of lamps, including a magnet mounted for movement adjacent to a plurality of magnetically operable switches, and a plurality of impedance elements so connected in a network with the switches that as the magnet is moved the switches are operated one or more at a time, and the impedance of the network varies.

The switches are preferably reed switches and the impedance elements are preferably resistors.

The magnet may be so fixed to a pivotally mounted arm that as the arm is rotated the magnet passes alongside the reed switches. In this arrangement the reed switches may be mounted radially, with respect to the pivot of the arm.

A magnetic shield plate with radial fingers between the reed switches may be used to improve the positive operation of the switches.

The network may consist of a potential divider connected to an electrical source, the divider being tapped at various points. The tapping points are then connected by way of the reed switches, and resistors, high in resistance compared with the potential divider, to an output terminal, there being one reed switch and one resistor connected to each tapping point.

In an alternative form the network may consist of resistors which are switched in or out of circuit by the switches to alter the resistance of the network. More than one magnet may be used to operate a number of switches simultaneously and in a preferred arrangement these magnets are mounted on a wheel and cooperate with fixed switches spaced around the periphery of the wheel.

Preferably the variable impedance apparatus is used to supply a control voltage to a dimmer drive circuit controlling dimmers in series with lamps to be controlled.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view, part cut away, of variable resistance apparatus according to the invention,

FIG. 2 is a circuit diagram for half of the apparatus of FIG. 1, and

FIG. 3 is a schematic diagram of a second form of variable resistors apparatus according to the invention.

Patented Jan. 26, 1971 In FIG. 1 a magnet 10 is mounted on an arm 11 which is pivoted on a shaft 12. The magnet can be moved adjacent to any one of six reed switches 13 to 18 by movement of a dolly 19, or operating lever, attached to the arm 11. The dolly is biased towards the centre position shown by a spring 20.

The reed switches are mounted on one side of a board 21, the other side of which carries a printed circuit interconnecting the electrical components of the apparatus.

Half of the switches are connected in the circuit shown in FIG. 2 which includes a potentiometer, made up of resistors R1 to R4 connected to a source having an output voltage V. The other switches are connected in a similar circuit. When the dolly 19 is pressed down switches in the circuit of FIG. 2 operate and provide a control voltage which increases in steps and appears at an output terminal 22. The brightness of lamps (not shown) controlled by the control voltage through dimmers and dimmer drive circuits (not shown) is thus raised. If the dolly is pushed upwards the brightness of the lamps is dimmed by the generation of a control voltage in the opposite sense.

The circuit of FIG. 2 also includes three equal resistors R5 to R7 each of which is of high resistance compared with the resistors R1 to R4 in series, a capacitor C to reduce any stray capacitive pick up on an output lead connected to the terminal 22, and a resistor R of high resistance compared with the resistors R. With all the switches open, that is with the dolly 19 in its centre position the resistor R which may be external to the apparatus, ensures that the output voltage is zero.

As the dolly 19 is pressed down, the magnet 10 first becomes adjacent to the switch 18 closing this switch. Next the magnet is equidistant the switches 17 and 18, and both these switches are closed. As the magnet approaches the switch 17, the switch 18 is released.

The full sequence switch operation is given in the following table where C denotes a closed switch, and P an open switch.

If for example it is assumed that the voltage V and the resistors R1 to R4 are such that the voltages at points 23, 24 and 25 are one, three and five volts, respectively, then the switch 18 is closed the output voltage is one volt. Similarly with the switch 17 closed the output voltage in three volts, and since the resistance of the resistor R5 equals that of the resistor R6, with both switches 17 and 18 closed the output voltage is two volts. Other output voltages are given in the table.

It is not essential in the circuit of FIG. 2 that resistor R be high in value compared with resistors R5, R6 and R7, nor that these latter be equal, nor that they be high in value relative to resistors R1, R2, 3 andR 4, although such relationships simplify calculations of required resistance values. Similar results may be obtained using resistors having values all of the same order; determination of the required resistance values is more complex but the current required from the source of voltage V may be reduced. Obviously and equally the resistor values may be chosen to give a non-linear control law.

A shield plate 26, of magnetic material, having radial fingers such as finger 27 is fixed to the board 21 to aid control of the flux path of the magnet 10. By choosing the proportions of magnetic material near to the switches, switch operation can be made more positive.

The variable resistance apparatus of FIG. 1 is shown as a unit which can be fitted into a frame containing other components of a lighting system. To this end the apparatus is mounted on a frame 28, and has a plug 29 for interconnection with the system.

In FIG. 3 four magnets to 13 are set in the periphery of a wheel 14. Four reed switches 15 to 18 are positioned near the wheel so that as the wheel is turned the switches close one after the other. The magnets and switches are such, and are so positioned, that when a magnet is between two switches both these switches are closed. As the wheel is turned first the switch 15 closes then the switch 16. The switch 15 is not then allowed to open since the magnet 11 is now in position to hold it closed. Thus the switches close one after another and remain closed until all the switches are closed. Further rotation of the wheel, in the same direction, then, of course allows the switches to open one after the other.

When switches 15 to 18 close they short circuit equalvalued resistors 19 to 22 respectively. Thus as the wheel is turned the resistance between terminals 23 and 24 falls in equal steps.

The reed switches may be of the type which are normally closed but which open in a magnetic field of a certain strength. The resistance between the terminals 23 and 24 would then be zero until the magnets were moved towards the switches, when the resistance would, of course, rise in equal steps.

In practice :many more than four magnets, reed switches, and resistors would be required in order to give a large control range in small steps.

The wheel 14 may of course be replaced by a slider which moves along a row of switches.

Instead of the resistors being in series, with a switch in parallel with each resistor, it is possible to connect the resistors in parallel, each resistor having a corresponding switch connected in series therewith.

The values of the resistors are not necessarily equal but could, for example, be arranged in a geometric progression. With three resistors of values R, Z and 4R arranged in series it would be necessary in order to obtain equal steps in the resistance of the network to arrange the switches to short the resistors out as follows:

all resistors shorted,

all but the first resistor shorted,

I all but the second resistor shorted,

only the third resistor shorted, and so on.

A variable resistance network as described with reference to FIG. 3 may be fed from a source of substantially constant current to produce a variable output voltage, or may itself control an output voltage developed across a fixed resistor connected in series with it and with a source of substantially constant voltage the output voltage in either case providing the control voltage for a dimmer drive circuit controlling dimmers operating on the lamps to be controlled.

What I claim is:

1. Apparatus for generating a voltage variable in steps by means of a manually-operable control member, comconnected in series with a separate resistor of resistance high compared with those making up the potential divider, and a magnet carried by the control member, the arrangement being such that the magnet is movable past the contact units of the array to operate each unit in turn and while between two contact units operates both units to give an output voltage lying between the voltages resulting from operation of each of the units separately.

2. A lighting control system including apparatus as claimed in claim 1 connected to supply a control voltage to a dimmer drive circuit controlling dimmers operating on the lamp or lamps to be controlled.

References Cited UNITED STATES PATENTS 2,666,175 1/1954- Seeger 323--97X 3,015,790 l/l962 lEisaman et al. 323--X 3,443,206 5/1969 Bradley 32379X ROBERT K. SCHAEFER, Primary Examiner H. I. H'OHAUSER, Assistant Examiner US. Cl. X.R. 32379 

